U.S. Pat. Nos. 3,977,018 and 4,037,311 disclose methods of manufacturing an infra-red detector element including the steps of forming a masking layer on part of a surface of a body of infra-red sensitive material, depositing metal on said masking layer and on a surface part of said body not covered by said masking layer, and removing said masking layer to lift away the metal thereon and leave the metal on said surface part for forming separate electrodes of said detector element. In both these patent specifications the masking layer is of photoresist which can be removed by dissolving in a suitable solvent such as acetone.
During operation, the resulting detector elements exhibit a high current density near the facing electrode-extremities defined by the removal of the masking layer. Sometimes these facing extremities of the electrodes are not sharply defined by the metal lift-off process; furthermore in these elements this high current density occurs adjacent part of the body surface between the electrodes, and the free charge-carrier recombination velocity may be high adjacent this surface in spite of passivation.
In both said patent specifications the photoresist masking layer is provided on part of a passivating layer formed at the surface of said body, for example by anodising. Before depositing the metal, the masking layer is used as a mask in a process for entirely removing the unmasked parts of the passivating layer so as to expose surface parts of the body for contacting by the electrodes. The process disclosed in U.S. Pat. No. 4,037,311 is a polishing process using a lapping cloth and a fine abrasive; the applicant has now found, however, that such abrasion can damage the surface of the infra-red sensitive material by introducing recombination centres within the sensitive area of the detector element. The anodic passivating layer is removed in U.S. Pat. No. 3,997,018 with a buffered hydrofluoric acid solution; however, the applicant has found that chemical etchant solutions also etch the passivating layer under the edge of the masking layer so that the metal electrodes left on removal of the masking layer may not adjoin the remaining passivating layer and may leave an area of unpassivated infra-red sensitive material where the high current density occurs in operation of these elements.
All these factors can adversely affect the low frequency (l/f) noise characteristics, and detectivity (D*) of detector element manufactured by these previous methods.